The sense by which sound is detected. In vertebrates the organ of hearing is the ear. In higher vertebrates variation in air pressure caused by sound waves are amplified in the outer and middle ears and transmitted to the inner ear, where sensory cells in the cochlea (see organ of corti) register the vibrations. The resulting information is transmitted to the brain via the auditory nerve. The ear can distinguish between sounds of different intensity (loudness) and frequency (pitch). In insects sound is detected by paired tympanal organs, found in various locations on the body depending on species. Each consists of a tympanum, analogous to the tympanic membrane of the ear, that vibrates in response to changes in air pressure. See tympanum.